Enhancing Well Productivity
It is often desired to increase the permeability of a subterranean reservoir that is penetrated by a well, so as to enable fluids to flow more easily into or out of the reservoir via the well. Fluids flowing into the well can be various fluids that are injected into the well for the purpose of enhancing the recovery and/or flowability of the desired hydrocarbons. Fluids flowing out of the well typically include the desired production fluids. Many rock formations that contain hydrocarbon reservoirs may originally have a low permeability due to the nature and configuration of the reservoir rock. Other reservoirs may become plugged or partially plugged with various deposits due to the flow of fluids through them, particularly drill-in fluids and/or completion fluids.
Matrix acidizing is a widely practiced process for increasing or restoring the permeability of subterranean reservoirs. It is used to facilitate the flow of formation fluids, including oil, gas or a geothermal fluid, from the formation into the wellbore; or the flow of injected fluids, including enhanced recovery drive fluids, from the wellbore out into the formation. Matrix acidizing involves injecting into the reservoir various acids, such as hydrochloric acid and other organic acids, in order to dissolve portions of the reservoir rock or deposits so as to increase fluid flow through the formation. The acid opens and enlarges pore throats and other flow channels in the rock, resulting in an increase in the effective porosity or permeability of the reservoir. In this sense, matrix acidizing refers to the treatment of homogeneous rock that is insufficiently porous.
Wormnholing
Wornholing is the preferred dissolution process for matrix-acidizing carbonate formations because it forms highly conductive channels efficiently. Hence, optimization of the formation of wormholes is the key to success of such treatments.
The ability to achieve increases in the near-wellbore permeability of formation and, therefore, the productivity of well by matrix acidizing in carbonate formations is related to fact that stimulation occurs radially outward from the wellbore. Because acid penetration (and the subsequent enhanced flow of oil or water) occurs through dominant wormholes that are etched in the rock by flowing acid, stimulation efficiency is controlled by the extent to which channels propagate radially away from the wellbore and into the formation. Under certain acidizing conditions, these channels may not propagate to a significant distance or they may not form at all.
Characterization of Wormholing Process
Numerous studies of the wormholing process in carbonate acidizing have shown that the dissolution pattern created by the flowing acid can be characterized as one of three types (1) compact dissolution, in which most of the acid is spent near the rock face; (2) wormholing, in which the dissolution advances more rapidly at the tips of a small number of highly conductive micro-channels, i.e. wormholes, than at the surrounding walls; and (3) uniform dissolution, in which many pores are enlarged, as typically occurs in sandstone acidizing. Compact dissolution occurs when acid spends on the face of the formation. In this case, the live acid penetration is limited to within centimeters of the wellbore. Uniform dissolution occurs when the acid reacts under the laws of fluid flow through porous media. In this case, the live acid penetration will be, at most, equal to the volumetric penetration of the injected acid. The objectives of the acidizing process are met most efficiently when near wellbore permeability is enhanced to the greatest depth with the smallest volume of acid. This occurs in regime (2) above, when a wormholing pattern develops.
The dissolution pattern that is created depends on the acid flux. Acid flux is the volume of acid that flows through a given area in a given amount of time, and is therefore given in units of velocity. (Units of l.sup.3 /l.sup.2.multidot.t=l/t). Compact dissolution patterns are created at relatively low acid flux, wormhole patterns are created at intermediate flux, and uniform dissolution patterns at high flux. There is not an abrupt transition from one regime to another. As the acid flux is increased, the compact pattern will change to one in which large diameter wormholes are created. Further increases in flux yield narrower wormholes, which propagate farther for a given volume of acid injection. Finally, as acid flux continues to be increased, more and more branched wormholes appear, leading to a fluid-loss limiting mode and less efficient use of the acid. This phenomenon has a detrimental effect on matrix stimulation efficiency, especially at the rate where branches develop secondary branches. Ultimately then a uniform pattern is observed. The most efficient process is thus one that will create wormholes with a minimum of branching and is characterized by the use of the smallest volume of acid to propagate wormholes a given distance.
Experimental research has shown that the process of wormholing depends mainly on three parameters: (1) surface reaction rate, (2) acid diffusion rate, and (3) acid flux. The surface reaction rate determines how fast acid reacts with carbonates at the rock surface. This rate is a function of the rock properties, such as composition and crystallinity, and of acid properties, such as concentration. The acid diffusion rate indicates how fast an acid molecule is transported from the bulk of the fluid to the rock surface. The diffusion rate is a function of the acid system. Both of these parameters are also a function of temperature. Depending on the reactivity of formation rock, either the surface rate or the diffusion rate may control the overall acid spending rate, though both are always in balance with each other. Wormholes form when the overall acid spending rate is balanced by acid transportation, i.e. the acid convection rate, or flux. Therefore, a wormhole is the result of dynamic process of acid reaction, diffusion and transportation.
Existence of Critical or Optimum Flux
The efficiency of the carbonate matrix-acidizing requires the maximum radial penetration at the lowest acid volume. The optimum flux is the one corresponding to this lowest volume. Extensive experimental investigation have shown the existence of an optimum acid flux that corresponds to the smallest amount of acid required to create wormholes of a certain length. Whenever the flux exceeds the optimum, a reduction in the flux will improve performance. Similarly, increasing fluxes that are less than optimum will improve performance. Injecting acid close to or above the optimum flux is very crucial to assure a successful carbonate acid treatment because of the risk of compact dissolution that may resulted from a slower acid injection. In other words, injecting acid at a high rate will ensure a success in matrix acid treatment, and injecting acid at the optimum flux rate will ensure the most efficient and successful matrix acid treatment. However, the optimum is a complex function of the formation properties, acid properties, and acidizing conditions so that there can be no simple rules as to whether slow or fast rates are best. The complexity stems directly from the range of dissolution patterns created by acid reaction with carbonates.
A few models have been developed to quantify wormhole growth in carbonate acidizing. However, these models were unable to either predict wormhole growth accurately or estimate the critical flux practically because they focused on only some of the acidizing mechanisms. Hence, there remains a need for a technique that will allow calculation of an optimum acid flux, and from that an optimum acid injection rate. The desired technique should be accurate and should rely on quantifiable parameters.